Arrhythmias, particularly sinus tachycardia, are often seen after open heart surgery or in the course of an acute myocardial infarct. Pharmacologic agents are often ineffective to control these arrhythmias, or may be contraindicated. Carotid sinus pressure and various types of pacing, such as rapid atrial pacing, paired or coupled pacing, and timed extra systoles, are often ineffective, especially in febrile states. We have seen in the OR during coronary revascularization and open heart procedures, that heart rate is easily and quickly controlled by slight local temperature changes of the SA node. We have evaluated this effect initially in an animal model, using an open chest preparation. Small cooling probes were designed to operate in conjunction with miniature thermocouples to cool and monitor the SA node. Initial data demonstrates the feasibility for varying SA node temperature in order to control heart rate within a clinically useful range. It is expected that complete data on this phenomena will lead to a simple technique for controlling arrhythmias which will have wide clinical use.